1. Field of the Invention
The present invention relates to video display systems. More particularly, the present invention relates to apparatus and methods for creating gray scale pixels on computer displays.
2. The Prior Art
Display controller devices, such as flat panel controllers use several different methods to develop gray scales on a display. A gray scale represents the different intensities of each of the Red, Green, and Blue pixels in the display. The gray scales for flat panel displays are typically created using a combination of analog techniques in the panel drivers. Known analog techniques include frame rate (temporal) gray scaling, and spatial gray scaling (dithering). Those of ordinary skill in the art understand that the word xe2x80x9cgrayxe2x80x9d is used even though it may relate to shades of Red, Green, or Blue.
Frame rate or temporal gray scaling has been described in the art. This technique trades off video frames for gray scales. It does this by modulating the pixel signal on and off on a per-frame basis. If a display has only two display colors (i.e., black and white), then a pixel may be turned on in every other frame to simulate a 50% gray value. The actual algorithm used in real-world applications is more complex due to the requirements to remove artifacts. This method for creating gray scale works quite well on computer interfaces since the data does not change much, and only contains a few colors in a typical application.
Spatial gray scaling is normally performed by trading pixel resolution on the screen for perceived shades of gray. If it is desired to generate 4 more shades on a display, then a two by two matrix is often used. Spatial gray scaling looks best on continuous tone images. These tend to have a limited bandwidth, but are much more sensitive to color. Spatial gray scaling tends to blur text, and is not recommended for computer graphics.
A typical prior-art display system is called upon to display both computer graphics and continuous tone images. Such a display system achieves colors by both spatial and temporal gray scaling. The result is typically a compromise between requirements for graphics images and requirements for continuous tone images. Gray scale implementations using combinations of both spatial and temporal gray scaling are preselected and fixed into display systems.
A method for developing gray scale pixels on a computer display according to the present invention comprises examining a run of pixels in the image content data for display on at least a portion of a computer display, determining whether the image content comprises computer graphics or continuous tone images, and choosing from among spatial and temporal gray scaling to apply to the image content data in response to whether the image content comprises computer graphics or continuous tone images.
According to one embodiment of the present invention, the number of unique colors in a selected run of pixels on the screen are counted to determine whether the image is a computer graphics image or a continuous tone image, and the gray scaling method is adjusted to best match the displayed image.
According to another embodiment of the present invention, the number of times that the same color appears in a selected run of pixels is counted. If the same color appears more than a threshold number of times, the image subject matter is probably computer graphics rather than a continuous tone image.
According to yet another embodiment of the present invention, the frequency content of a selected run of pixels may be determined. If the frequency content of the selected run of pixels is high (e.g., above about 5 MHz) the content is probably computer graphics text. On the other hand, if the frequency content of the selected run of pixels is low (e.g., below about 2 MHz) the content is probably a continuous tone image.